In-situ estimate of submesoscale horizontal eddy diffusion

Geophysical Research Abstracts
Vol. 15, EGU2013-8237-2, 2013
EGU General Assembly 2013
© Author(s) 2013. CC Attribution 3.0 License.
In-situ estimate of submesoscale horizontal eddy diffusion coefficients
across a front
Francesco Nencioli (1), Francesco d’Ovidio (2), Andrea Doglioli (1), and Anne Petrenko (1)
(1) Aix-Marseille University, Mediterranean Institute of Oceanography (MIO), 13288, Marseille, Cedex 9, France ; Université
du Sud Toulon-Var, CNRS-INSU/IRD UM 110 ([email protected]), (2) Laboratoire d’Océanographie et du
Climat: Experimentation et Approches Numeriques (LOCEAN), IPSL, Paris, France
Fronts, jets and eddies are ubiquitous features of the world oceans, and play a key role in regulating energy budget,
heat transfer, horizontal and vertical transport, and biogeochemical processes. Although recent advances in computational power have favored the analysis of mesoscale and submesoscale dynamics from high-resolution numerical
simulations, studies from in-situ observations are still relatively scarce. The small dimensions and short duration
of such structures still pose major challenges for fine-scale dedicated field experiments. As a consequence, in-situ
quantitative estimates of key physical parameters for high-resolution numerical models, such as horizontal eddy
diffusion coefficients, are still lacking.
The Latex10 campaign (September 1-24, 2010), within the LAgrangian Transport EXperiment (LATEX), adopted
an adaptive sampling strategy that included satellite data, ship-based current measurements, and iterative Lagrangian drifter releases to successfully map coherent transport structures in the western Gulf of Lion. Comparisons with AVHRR imagery evidenced that the detected structures were associated with an intense frontal feature,
originated by the convergence and subsequent stirring of colder coastal waters with warmer open-sea waters.
We present a method for computing horizontal eddy diffusion coefficients by combining the stirring rates estimated
from the Lagrangian drifter trajectories with the shapes of the surface temperature and salinity gradient (assumed
to be at the equilibrium) from the ship thermosalinograph. The average value we obtained from various sections
across the front is 2.5 m2 s−1 , with horizontal scales (width of the front) ranging between 0.5 and 2.5 km. This is
in line with the values commonly used for high-resolution numerical simulations. Further field experiment will be
required to extend the results to different ocean regions and regimes, and to thoroughly test the robustness of the
equilibrium hypothesis.
Remote sensed measurements of sea surface temperature and elevation could also be used to compute fine-scale
horizontal eddy diffusion coefficients over larger areas and for different ocean regions. However, the coarse resolution of current sea surface topography observations, and their unreliability over coastal regions, represent important
limitations for this type of application. The velocity fields provided by the SWOT mission will allow to retrieve
accurate high-resolution stirring rates across the ocean. Combining these rates with remote-sensed SST gradients
will make possible to extend our analysis and investigate patterns and variability of submesoscale horizontal eddy
diffusion at the global scale.